In electrophotographic light-sensitive materials widely used in recent years, there are used selenium, cadmium sulfide, zinc oxide, and the like as inorganic photoconductive substances; and various photoconductive polymers including poly-N-vinyl carbazole and polyvinyl anthrathene as organic photoconductive substances. However, these substances are not satisfactory in film-forming properties and flexibility and have drawbacks that they likely cause cracks or exfoliation when left in the film state. In order to compensate these drawbacks, it has been proposed to add a plasticizer or a binder, but electrophotographic properties such as sensitivity or residual potential are reduced though the flexibility is improved and, accordingly, it has been extremely difficult to obtain a practically useful light-sensitive material. Low-molecular weight organic photoconductive compounds have themselves no film-forming ability but can be formed into films capable of providing light-sensitive materials excellent in film-forming properties and flexibility by appropriately combining with a high-molecular weight binder such as polyester resins, polyvinyl chloride resins, and polycarbonate resins.
In addition to the system of using these photoconductive substances, there are proposed various methods of obtaining two functions of the photoconductive substance, that is, generation of charge carriers and transportation of generated charges, by separate organic compounds as described in, for example, U.S. Pat. No. 3,791,826. In this method, it is possible to obtain electrophotographic light-sensitive materials having high sensitivity by the combination of substances having a high generation efficiency for charge carriers with substances having a high charge transporting ability. However, it is not always possible to simultaneously attain those various properties required for the electrophotographic light-sensitive material, that is, high surface charge, high charge retention ability, and high photosensitivity, with substantially no residual potential. In order to obtain practically useful light-sensitive materials having such various properties, it is important to provide high generation efficiency of charge carriers in the charge generating substance and rapid transportation of charge carriers in the charge transporting substance, as well as efficient injection of electric charges from the charge generating substance to the charge transporting substance, i.e., efficient injection of charge carriers from the charge generation layer to the charge transport layer in the laminate-type light-sensitive material. Although it has been attempted to explain the injection efficiency by means of a correlationship with the ionization potential of the charge transporting substance, it still lacks in generality, and explanation cannot be made systematically for the entire charge transporting substance. The charge injection varies depending on the characteristics of the interface between the charge generating substance (or charge generation layer) and the charge transporting substance (or charge transport layer) and is not uniform among various kinds of substances.
On the other hand, while several compounds have already been known from 1,1,4,4-tetraphenyl-1,3-butadiene derivatives with which this invention is concerned, only 1,1,4,4-tetraphenyl-1,3-butadiene is known to be useful as the electrophotographic light-sensitive material (as described in, for example, M. Kleinerman et al., J. Chem. Phys., 37, 1825 (1962) and Japanese Patent Application (OPI) No. 24248/77). (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) However, 1,1,4,4-tetraphenyl-1,3-butadiene and known alkyl-, alkoxy-, or halogen-substituted 1,1,4,4-tetraphenyl-1,3-butadiene derivatives thereof are extremely of low sensitivity and poor in solubility in binder polymers. Further, although only 1,1,4,4-tetrakis(p-dimethylaminophenyl)-1,3-butadiene has been known so far as alkylamino group-containing derivatives (as described in C. E. H. Bawn et al, Chem. Commun., 599 (1968)), this compound has no electrostatic charge retention ability and cannot substantially be used as the light-sensitive material.
In electrophotographic light-sensitive materials having function-separated type light-sensitive layers, though high sensitivity can possibly be obtained by selection and combination of substances having respective functions as described above, the conventional electrophotographic light-sensitive materials of this type have a drawback that when used repeatedly according to the electrophotographic process, the ability to recover original charging properties is reduced or the light sensitivity is decreased to thereby shorten the life time of the light-sensitive material. That is, when the actual process of electrophotography such as charging, exposure, and cleaning is repeated for many times, one or more of light-fatigue phenomena are brought about such as fluctuation of the surface charge after charging, reduction in charge retention ability, reduction in light sensitivity, increase in residual potential, and the like, which greatly reduce the performance of electrophotography and bring about a severe problem for practical use.
The present inventors have made extensive investigations to various compounds which are suitable for the preparation of an electrophotographic light-sensitive material of a further higher performance among 1,1,4,4-tetraphenyl-1,3-butadiene derivatives having one or two di-lower alkylamino groups unexpectedly possess excellent properties as an electrophotographic light-sensitive material, that is, satisfactory solubility, high sensitivity with low residual potential, and high durability with less light fatigue even after repeated use, which finding lead to this invention.